1. Field of the Invention
The present invention relates to a photo detection device such as a CMOS image sensor.
2. Description of the Related Art
In recent years, CCDs (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) image sensor have been used for various image pickup apparatuses such as digital still cameras or digital video cameras.
As a merit of using the CMOS image sensor, the same manufacturing line as that for other general chips is used and it can be packaged into a single chip together with peripheral functions. In this respect, the CMOS image sensor can be driven at lower voltage than CCD and the CMOS image sensor consumes less power than the CCD.
Each pixel of a CMOS sensor has a structure including a photodiode and a switch using MOSFETs. That is, the sensor has a matrix of photodiodes, each of which has a switch attached thereto, and the electric charge of each pixel is read out by operating these switches one by one. For example, Nonpatent Document 1 discloses a pixel circuit of such a CMOS image sensor.
FIG. 1 is a circuit diagram showing a structure of a pixel circuit 200 of a conventional CMOS image sensor. This pixel circuit 200 includes a photodiode PD, a reset transistor M11, an amplifying transistor M12, and an output transistor M13. The reset transistor M11, the amplifying transistor 12, and the output transistor M13 are all n-channel MOSFETs. The reset transistor M11 and the photodiode PD are connected in series between a supply voltage Vdd and a ground voltage GND. A source terminal of the reset transistor M11 is connected to the photodiode PD, and the supply voltage Vdd is applied to a drain terminal thereof. A reset signal RST is inputted to a gate terminal of the reset transistor M11.
A cathode terminal of the photodiode PD, which is connected with the reset transistor M11, is connected to a gate terminal of the amplifying transistor M12. The supply voltage Vdd is applied to a drain terminal of the amplifying transistor M12, and a source terminal thereof functions as a source follower amplifier, which is connected to a drain terminal of the output transistor M13. A source terminal of the output transistor M13 is connected to a data line LD, which is provided for each column of the CMOS image sensor.
In a pixel circuit 200 structured as described above, when a reset signal RST inputted to the gate terminal of the reset transistor M11 goes to a high level, the reset transistor M11 turns on, thereby applying the supply voltage Vdd to the photodiode PD and charging the cathode terminal thereof with the supply voltage Vdd. Then, the reset transistor M11 turns off. In this state, if light strikes the photodiode PD, a photocurrent will flow, and electric charge stored at the cathode terminal of the photodiode PD will be discharged. At this time, the voltage at the cathode terminal of the photodiode PD changes with the light intensity and the storage time.
After a predetermined storage time has passed, setting a selection signal SEL to a high level turns the output transistor M13 on and a voltage corresponding to the amount of light received by the photodiode PD is outputted, so that the amount of light received by each pixel circuit can be read by an external circuit.
[Nonpatent Document 1] Ikebe et al., “Evaluation of a Functional Initializing for a CMOS-Image Sensor”, Technical report of IEICE, The Institute of Electronics, Information and Communication Engineers, September 2003, vol. 103, No. 298, p. 19-24.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-197362.
[Patent Document 2] Japanese Patent Application Laid-Open No. 2004-363666.
Here, reviewed is the dynamic range of a pixel circuit of a conventional CMOS sensor described in FIG. 1. As described above, in detecting the amount of light received by each pixel, the photodiode PD is charged with the power supply voltage Vdd, the electric charge stored at the cathode terminal of the photodiode PD during an exposure period is discharged, and the remaining charge amount is converted into voltage to measure the amount of light received. Consequently, if a strong light enters the photodiode PD and the remaining charge amount becomes zero within the charge storage time, the pixel circuit 200 can no longer detect the amount of light having entered the photodiode PD.
Conversely, if the storage time is shortened, the remaining charge amount will not be zero. Thus, strong light can be detected, but if a weak light enters in this state, then it cannot be detected. As stated above, with the conventional pixel circuit 200, the dynamic range is subject to limitation by the amount of initial charge stored at the cathode terminal of the photodiode PD in a reset state. The conventional technique for widening the dynamic range has been through logarithmic conversion or changing the storage time and gain as described in Patent Document 1 or Patent Document 2.
However, a type of circuit which is so-called an active pixel sensor as shown in FIG. 1 where the storage time is varied has a problem that the shorter the minimum storage time is made, the more the power consumption will be for driving the circuit at high speed. Where the gain is varied, it is inevitable that the circuit be made larger in scale if the gain is to be set high.